Whispering-Gallery Mode (WGM) optical microresonators have emerged as rich experimental platforms for quantum optics, photonics, and sensing. In such systems, propagating light is confined to a microstructure and made to repeatedly probe the same volume. Scrupulous minimization of attenuation and scattering translates into tremendously high quality (Q) factors, exceeding 1010 for microsphere resonators. (See Gorodetsky, M. L.; Savchenkov, A. A.; Ilchenko, V. S.: Ultimate Q of optical microsphere resonators. Opt. Lett. 1996, 21, 453-455.) Simultaneously, the propagating mode may be evanescently coupled to the microstructure's immediate environment, leading to repeated interaction with this environment similar to cavity ring-down spectroscopy, though on a substantially smaller length scale. The result is a highly sensitive probe of local environment. Microresonators are also sensitive to the presence of absorbers, and even sensitive in a label-free capacity to the presence of non-absorbing analyte species through differences in the real part of the complex refractive index between the analyte and the surrounding medium (termed the “reactive mechanism”). In particular, toroidal microresonators possess the highest ratio of Q to propagating mode volume. (See Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J.: Ultra-high-Q toroid microcavity on a chip. Nature 2003, 421, 925-928 and Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J.: Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip. Appl. Phys. Lett. 2004, 85, 6113-6115.) This combination has made them particularly suited for applications in nonlinear optics.
In previous single-particle detection experiments, the reactive mechanism shifts the resonance position upon analyte binding. Though the shift is small, it is resolvable due to the narrowness of the linewidth itself. Various methods have been applied to stabilize the optical properties of the resonator, eliminate drift, or use internal standards to minimize spurious resonance position fluctuations in attempt to increase the resolution and decrease the minimum detectable object size. However, even with these improvements, typical detectable objects are in the ˜10-100 nm size range, such as nanoparticles and virus particles. (See Zhu, J. G.; Ozdemir, S. K.; Xiao, Y. F.; Li, L.; He, L. N.; Chen, D. R.; Yang, L.: On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator. Nature Photon. 2010, 4, 122-122; Lu, T.; Lee, H.; Chen, T.; Herchak, S.; Kim, J. H.; Fraser, S. E.; Flagan, R. C.; Vahala, K.: High sensitivity nanoparticle detection using optical microcavities. Proc. Natl. Acad. Sci. U.S.A 2011, 108, 5976-5979; He, L. N.; Ozdemir, K.; Zhu, J. G.; Kim, W.; Yang, L.: Detecting single viruses and nanoparticles using whispering gallery microlasers. Nature Nanotech. 2011, 6, 428-432; and Vollmer, F.; Arnold, S.; Keng, D.: Single virus detection from the reactive shift of a whispering-gallery mode. Proc. Natl. Acad. Sci. U.S.A 2008, 105, 20701-20704.) Additionally, no chemical information regarding the identity of the adsorbed species is obtained.